Problem: The scale on a map is 5cm : 4km. If the distance between two cities is 12km, how far apart in cm are the two cities on the map?
The scale means that every 5cm on the map corresponds to an actual distance of 4km. An actual distance of 12km is the same as 3 $\cdot$ 4km. The distance between the two cities on the map is 3 $\cdot$ 5cm, or 15cm.